I brewed a Porter about a month ago. OG was 1.054. Nottingham dry yeast, fermentation was slow to take off, about 36 hours, but went pretty good for a few days, then slowed considerably, but continued to bubble very slowly for about 3 weeks, even in the secondary. When bubbling stopped, I siphoned to an ale pail for storage till my keg is empty, FG is now 1.022. This was a kit, but they didn't suggest what the FG should be. Does this seem low enough, or should I do something to try to get it lower?

Based on your OG reading, Robust Porter should be down more to 1.016-1.012. Should really finish at between 4.8%-6.5%. If you believe it it is finished fermenting, you have ended up at 4.2% ABV. It will be fine.
Cheers,
Brian

If it stopped bubbling, it's probably done, but 1.054-1.022 is only 54% attenuation, which is low for Nottingham, so I'd still be a little concerned if I was bottling. Since you are kegging, it will just be a bit sweet, but at least you don't have to worry about bottle bombs.

Since it's been a few more days, check the gravity again and see if it's still dropping. Sometimes it will still slowly creep down.

If this is an extract beer, especially if it used much dark extract, it may simply be done. Depending on the brand of extract, I had difficulty getting my extract beers much below 1.018-1.020 and dark extract tends to be less fermentable.

That's good to hear. Very unusual for Nottingham to be so slow. I wonder if you just got a pack that had been abused at some point. Perhaps it spent a couple days in the back of a hot truck during shipping, or an afternoon in the sun on a loading dock.